Finding a wiring short

I've got a 2003 Protege that has a short in the wiring harness somewhere (I hope). What's the best and easiest way to figure out where the short might be? The fuse is for the ECU and all the wiring and sensors that go with it. Occasionally I've been able to move the harness and get it to start briefly but now I'm down to just blowing fuses each time I turn the key on. I'm horrible with electrics so be gentle with me.

Thanks

Douse it with gas and see where the fire starts?

But seriously...There's no easy short cut. Find a wiring diagram and figure out which wire is connected to the fuse that's blowing. Start tracking it though all the nooks and crannies and look for obvious abrasion, rubs, cuts, etc. Look for any aftermarket mounting screws that were mistakenly run into the wiring by a previous owner while mounting his cell phone holder (or whatever.)

TBH, on a car that new I've never seen a harness just develop a short unless something had been done to really harm the wires. They've gotten pretty smart about protecting them against vibration and wear.

Take one lead of a volt meter and put it into one of the wires of the harness connector that is in question, then put the other meter lead to the nearest ground (a ground that the wire you are testing would NOT attach to.) if you have continuity, the wire is ground out here.

If you are looking for an internally broken wire, do the same, but attach the ground lead to the actual spot where the wire runs to (like a ground or a sensor.) if you DONT have continuity, you have a bad wire. A wire should look like this _ and not _ In other words, no breaks internally so that electricity can pass through the wire and do its job.

1. Grab a wiring diagram for the vehicle, and an digital multimeter (available at HF for like 3 bucks usually). Set it to resistance, or ohms.
2. start with the one fuse you are working on. I would go from the simplest circuits and work toward the most complicated.
3. Check resistance between the fuse and ground. Try it with ignition key off, key on, and key start. If you get low readings (basically any readings) you have confirmed your short.
4. Next step is to go a smaller distance. For example, if this connects to pin 41 of the ECU, check resistance between the pin 41 of the ECU and ground. If you still have a resistance reading, the short is still in between your two test points. If you no longer get a resistance reading, you just passed your short.

This can be a hugely painful process. Here's how to not have to do it at all:

1. Way too many times I have heard of an electrical problem with a component being misdiagnosed as a 'short somewhere'. Wiring rarely fails unless it is in a few specific conditions (more info below), but components frequently fail and cause weird outcomes. Unplug one component, and then plug in the fuse. If the fuse doesn't blow with a component removed, that may be a sign, try a known good component.
2. If you know your components are all good, then look into problems with the wiring itself. To check for problems with wiring, look for wires with physical damage. (broken wires, cut or torn shielding). Check areas that may have high heat or areas where wires bend back and forth frequently.

M^^^ or do it the easy way. Your call

Stick a 22 shell in the fusebox and follow the smoke after it fires.

In reply to ultraclyde:

There's been an engine swap in the cars past and a lot of the sheathing has been removed. Found one ground wire that looks like it got cooked and the insulation is falling off of it.

Robbie wrote: This can be a hugely painful process. Here's how to not have to do it at all: 1. Way too many times I have heard of an electrical problem with a component being misdiagnosed as a 'short somewhere'. Wiring rarely fails unless it is in a few specific conditions (more info below), but components frequently fail and cause weird outcomes. Unplug one component, and then plug in the fuse. If the fuse doesn't blow with a component removed, that may be a sign, try a known good component. 2. If you know your components are all good, then look into problems with the wiring itself. To check for problems with wiring, look for wires with physical damage. (broken wires, cut or torn shielding). Check areas that may have high heat or areas where wires bend back and forth frequently.

^ start there.

But yeah, it sucks. The last time I had a similar issue the 02 sensor line had melted itself against the exhaust manifold of my friend's Jeep. Give that a good gander before you start in with the meeter just in case.

The melted looking ground wire, I don't think is the issue. It's already grounded so it touching another ground shouldn't affect it any. But if it and an adjacent wire are both melted and touching then that's another story...

Good luck man,

-Bill

Oh good, an actual short. Knurled-approved.

In the shop, the first thing I'd do is find out what goes on that circuit and unplug everything and see if the fuse still blows.

If it's not something like that, I'd see what major chassis harness connectors are in the way and unplug those and see if it stops blowing fuses. Basically identifying where the wiring chaed through by process of elimination.

If that isn't an option, I'd get out yet olde scope and amp clamp and amp clamp each wire coming out of the fusebox from that circuit... back before I had such luxury, I'd just cut the wires one at a time until the fuse would stop blowing.

Any time a customer would abandon a car or have a car scrapped, we would habitually yank all of the fuses/relays. Have hundreds of used fuses lying around...

Actually... the FIRST thing I'd do is look for obvious wire damage where wiring harness goes past sheetmetal. And then I'd go to iatn (International Automotive Technician Network, or I Ain't Tested Nothin') and see if there is a pattern failure...

Knurled wrote: Oh good, an actual short. Knurled-approved. In the shop, the first thing I'd do is find out what goes on that circuit and unplug everything and see if the fuse still blows. If it's not something like that, I'd see what major chassis harness connectors are in the way and unplug those and see if it stops blowing fuses. Basically identifying where the wiring chaed through by process of elimination. If that isn't an option, I'd get out yet olde scope and amp clamp and amp clamp each wire coming out of the fusebox from that circuit... back before I had such luxury, I'd just cut the wires one at a time until the fuse would stop blowing. Any time a customer would abandon a car or have a car scrapped, we would habitually yank all of the fuses/relays. Have hundreds of used fuses lying around...

That was a VERY good suggestion. Started unplugging things and low and behold it was the circuit going to the rear 02 sensor. Ran fine with it unplugged. Traced the harness back and found nothing unusual. Got to the 02 sensor wire itself and found it draped across the exhaust. Yup, burned right into the power wire. Replaced it and we were off to the races.

Thanks everyone for their suggestions. Very helpful.